Coating metals



Oct. 11, 1960 H. DORNER comma METALS Filed Sept. 10, 1957 F'IG.1

" FIG. 2.

iNVENTOR j ATTORNEY United States Patent C) we COATING NIETALS Henrik Dorner, London, England, assignor, by mesne assignments, to Intel-chrome S.A., Geneva, Switzerland, a Swiss company Filed Sept. 10, 1957, Ser. No. 683,014

Claims priority, application Great Britain Sept. 21, 1956 3 Claims. (Cl. 117-107) This invention relates to a process of coating a metal with another metal by diffusion and is particularly concerned with a process of the kind in which the difiusion is effected in a closed container in which a ferrous metal to be coated, is heated in the presence of a halide of chromium which is the coating metal, and thereafter the container is cooled.

In general the halide is produced in the container by the reaction of the coating metal with a halide, e.g. an ammonium halide such as ammonium iodide.

It will be understood that the term metal as used herein includes alloys.

In carrying out a process of the aforesaid kind the container is usually either a completely closed and gastight box or a sealed box, that is to say a box provided with a channel to receive the lid and sealed by a substance such as glass which is fusible at the operating temperature but solid at room temperature.

It has been found according to this invention that in a process of the aforesaid kind significant changes of pressure take place within the container. These are as follows:

(a) The positive pressure rises to a maximum value before the container reaches the processing temperature, subsequently falls off to atmospheric pressure and then steadily falls below atmospheric pressure while the temperature of the container is still rising.

(b) This negative pressure or partial vacuum eventually reaches a maximum value.

(c) In a box which is completely welded and gastight, this partial vacuum once reached remains steady, while in a sealed box the partial vacuum is smaller and discontinuous inasmuch as the pressure oscillates between atmospheric and negative pressure in periods of the order of minutes.

It has also been observed that when the negative pressure arising during cooling of the container is balanced by passing an inert gas, e.g. nitrogen or argon, into the box, in the case of the completely welded gas-tight box neither the articles to be coated nor the treatment mixture used is oxidised; in sealed boxes, however, a considerable percentage of the articles is oxidised, particularly those situated near the walls and on the top, and the loss of coating metal in the top layers of the treatment mixture due to oxidation is very considerable. This is due to the periodical intake of furnace gases as shown by the loss of negative pressure, the residual oxygen causing the oxidation.

contain various carbon, sulphur and other compounds (according to the type of fuel used) and that these compounds become adsorbed by the contents of the box,

2,955,957 Patented Oct. 11, 1960 contaminate the treatment mixture and inhibit the reac tions.

It has been found that these disadvantages can be overcome by the process of the present invention, according to which a process of the aforesaid kind is carried out whilst allowing or causing an inert gas to enter the container at least during the cooling of the container.

This can be effected by using a container connected by means of a tube to one or more pressure gauges and to a source of one or more inert gases by means of which the partial vacuum produced during the process is held within the sealing power of the sealing medium used and by means of which the vacuum arising in the container during cooling can be compensated. Either the same gas may be used for both purposes, e.g. nitrogen or argon of the vapour of an ammonium halide, iodine, or of an organic iodo or chloro compound such as iodoform or chloroform, or one gas for the period of the furnacing operation, e.g. an ammonium halide vapour which dissociates into nitrogen, hydrogen and a halogen or halide, and another gas for the period of cooling.

A further advantage of such a system is that any change from the normal pattern of pressure changes gives an early warning of a fault, e.g. if the pressure does not rise in the usual manner at the beginning of the treatment cycle this would indicate that the box is leaking,

i.e. that a cracked box is being used, and the box can be withdrawn from the furnace before any damage to the contents has arisen. Further, should the pressure become abnormal at a latter stage of the treatment cycle, e.g. should no partial vacuum arise, this would indicate that the box wall has become perforated in the course of processing and steps could be taken immediately to interrupt the process, and by means of creating a small positive pressure during the cooling with the help of, e. g. nitrogen, the contents of the box could be salvaged and also valuable furnacing time saved. This would apply equally to a completely welded gas-tight box and to a box assembly incorporating a sealing medium.

Alternatively, the inert gas can be supplied to the container during the cooling period of loading the container for cooling into a vessel or enclosure in which the atmosphere of an inert gas can be maintained during the whole period of cooling. This vessel or enclosure may be a bell which is lowered on to the container or it may be a shed with a gas-tight door into which an inert gas such as nitrogen, ammonia or argon may be passed.

It has further been found that a process of the aforesaid kind may be improved by varying the pressure inside the container by bringing it up to atmospheric pressure or even to super-atmospheric pressure.

Chromium or a chromium alloy is used as the coating metal.

The metal to be coated is a ferrous metal such as iron or steel.

In the accompanying drawings:

Fig. 1 shows a view partly in section of the apparatus used in Example 1; and

Fig. 2 shows a view partly used in Example 2.

The following examples illustrate the invention: Example 1 is given for comparative purposes only and is designed to show the pressure changes which take place during the process.

in section of the apparatus EXAMPLE 1 parts by weight of ferrochrome and 1 part by weight of,

kaolin to which was added 0.1% of its weight of iodine in the form of ammonium iodide in a steel box 7 which of the box after it had been packed. The ferrochrome contained about 65% of chromium, 0.1% ofrcarbon and about 4% of silicon, the rest being iron:

Connection between the box atmospheresand two pressure gauges 9 and 10 situated outside the furnace was established by means of a steel tube 11' which passed through the front of the box and ended just below the lid. Prior to loading into the hot'furnace the whole .assembly was tested for gas tightness under pressure.

Of the two pressure gauges one, 9 was calibrated in lbs. per sq. in. to read the larger pressures that-are likely to arise at the beginning of processing; the other, 10,

was calibrated in inches of water and capableflof indicating pressures both above and below atmospheric pres sure. By a suitable arrangement of valves 12 and 13 it was possible to operate either of the gauges only, and also. to' relieve the box of excess pressureshould it arise.

The box was loaded into a furnace 14 standing at 600 Cland' set for a processing temperature of 980 C. The tube 11 mentioned above fitted into a groove 15.

furnace into'which the box was loaded, these tempera tures being room temperature, 410 C. and 800 C.

In the three runs the maximum positive pressure b- 7 served was the same and amounted to 15" water gauge the difference being only the time required to reach this (the, longest time when-stanting ina cold furnace, the a shortest time when starting in a hot furnace). The

' pressure dropped to atmospheric pressure in each case in about the same timerv Further pressure. drops occurred in eachcase, to the nearly identical value of minus 14"-15" water gauge, in about the same time. In each case, having reached the minimum Value of minus 15 water gauge there was'a continuous pressurechange,.the pressure suddenly rising to atmospheric pressure (the dial of the gauge moving back to zero) and then dropping again to the previous value, the cycle repeating itself approximately every 2 minutes. The

' table below shows the difference in time and identity in under'the'furnace door 16, the instruments were about two feetaway from the furnace door and were'shielded' against heat by a shield 17. V V

The pressure rose to 7 lbs/sq. in. wi-thin lS'minutes.

Samples of'the gas were taken from tube 18 controlled by valve 19 resulting in a pressure drop to 6 lbs/sq. in.

. The pressure remained steady for 90 minutes during which the furnace temperature had ,risen to 900 C. At this stage 'the pressure started'to fall off, and15 min utes later, i.e. 2 hours after the beginning of the whole 7 operation, the pressure gauge dial ,was' standing at zero (i.e; atmosphericpressure) while the furnace tempera ture had risen to 925 C. From' now on the pressure within the box dropped steadily below atmospheric-pressure'and'within 40 minutes the water gauge dial showed a value of minus 15 inches, the box having a temperature of 980 C. as determined by an optical pyrom'eter. The pressure continued todrop off, in otherwordsthe partial vacuum continued to increase, at first just as steeply and later'more slowly, until 2. hours laterthe gauge showed minus 31 inches, While the temperature remained steady at 980 C. The pressure gauge remained steady at this value forthe remainderofthe'op eration, i.e. another-4 hours.

After opening the box which was severely'distorted it was found that both the mixture and the treated articles were free from oxidation. The chromium content of the mixture taken from the top of the box was substantially the same (41.6%) as the chromium content of a sample taken from the centre of the'box (41.8%). All the articles treated were of uniform silvery-gray colour, the'thickness of the chrome-alloyed surface layer,

7 insoluble in nitric acid, being 0.0025" on all thesamples taken from the different positions in the box:

EXAMPLE 2' Referring to Fig. 2 of the drawings-flat sheets 1, perforated sheets 2, circles 3, screws 4 and nuts 5 all of mild steel were packed in a mixture 6 consisting of 2v through the front of the lid 22 into the gap between the" lid andthe inner box 20-andconnected to gauges. 9 and 10 -as.in Example 1. Sealing of. the box'was effected by. means of a fusible glass 23; Three runs'were made in which the only difference was the temperatureof-the pressure values.

Table Time taken to reach Times taken for Time taken When box is plus 15" Water gauge pressure to drop for pressure loaded into a pressure and temperto atmospheric to drop to furnace atature of the furnace and temperature minus14"-15 V at that time of furnace watergauge Room temp. 3 hours, 5 minutes, 1 hour, 55 minlhour, 25

910 C. utes, 1,010"). minutes. 410 C 2 hours, 20 minutes, 2 hours, 25 miu- 1 hour; 25

0 C. utes, 980 0. minutes. 800 C 1 hour, 980 0 1 hour, 40 min- 1 hour, 20

' utes, 980 0. minutes.

After opening the box in each case it was found'that the mixture taken fromthe top of the box up to a depth of about one inch from the surface was poorer in chromium than the sample taken from the. centre ofthe box. The loss of chromium by means other than due' to 1 diffusion into'the articles treated was essentially caused by the conversion of metallic chromium into chromiu'm oxide by oxygen which entered the box during and after furnacing, and any drop in the acid soluble chromium content was a measure of'the oxidation suffered by the mixture; The mixtures were analysed for chromium content '(chromium soluble in sulphuric acid) and it was found in one case that scrapings taken from the surface of the mixture in the box contained 8.75% Cr, the top layer up toa depth of 1 inch 37.60% Cr, the mixture" from the centre of the box 41.24% Cr. In other words'there was, on the surface, a loss of. of metallic chromiumand up to a depth of 1 inch a loss of nearly 10% of metallic chromium as compared with the centre of the box. Similar results were obtained in the two other runs.

With regard to the treated articles it was found that in general those near the wallsof the box and1on the top were frequently only partly chromium-diffused on their surface, showing barerpatches 'undertest; These bare patches were the'consequence of the intake of furnacing gases in the box. .Further, in the same positions,

articles frequently showed considerable discolouration.

due'to oxidation. In either case they must be considered as rejects and the percentage of such rejects'was of the order of 10-20%. The thickness of the chrome-alloyed surface layer insoluble in nitric'acid' was 0.0027 generally, except of course on parts which were faulty or near to faulty parts during treatment; onthese latter the coating thickness varied and was as low as 0.001". When the runs were repeated but'admitting nitrogen into the box through tube 24 controlled by valve 25 during the treatment and cooling in a nitrogen atmosphere, similar results to those of Example 1 were obtained,

What I claim is: 1. A process-for coating ferrous metalwith chromium by. diffusion, which comprises carrying out. the diffusion in a closed container having an inlet for inert gas without any outlet therefor by heating said container'and its contents in a furnace and withdrawing the container from the furnace when heating is completed, and admitting the inert gas into the container at least after completion of the heating as soon as pressure within the container begins to fall below atmospheric pressure.

2. The process of claim 1 in which the inert gas is argon.

3. The process of claim 1 in which the inert gas is nitrogen References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS FOR COATING FERROUS METAL WITH CHROMIUM DY DIFFUSION, WHICH COMPRISES CARRYING OUT THE DIFFUSION IN A CLOSED CONTAINER HAVING AN INLET FOR INERT GAS WITHOUT ANY OUTLET THEREFOR BY HEATING SAID CONTAINER AND ITS CONTENTS IN A FURNACE AND WITHDRAWING THE CONTAINER FROM THE FURNACE WHEN HEATING IS COMPLETED, AND ADMITTING THE INERT GAS INTO THE CONTAINER AT LEAST AFTER COMPLETION OF THE HEATING AS SOON AS PRESSURE WITHIN THE CONTAINER BEGINS TO FALL BELOW ATMOSPHERIC PRESSURE. 